1. Field of the Invention
This invention relates to circuit interrupters which respond to ground faults and in particular to such circuit interrupters with immunity to false tripping caused by transients such as wide band noise induced by load related switching phenomenon.
2. Background Information
Ground fault interrupters include ground fault circuit breakers, ground fault receptacles, and even cord mounted ground fault protection devices. Ground fault interrupters may be troubled by false tripping, even though they pass all present industry standards. One cause of false tripping is disconnection of the power to inductive appliances, particularly by unplugging the appliances.
Examples of these appliances include electric shavers, high intensity lamps, and small cooling fans, such as are used for cooling electronic equipment. Unplugging these appliances generates an arc between the plug and the receptacle, resulting in the superimposition of several volts of wide band noise onto the power line. Due to the wide band nature of the noise, even a very small stray coupling capacitance will couple the noise from the power line conductor into the ground fault circuit, causing a false trip.
A typical ground fault interrupter includes an operational amplifier which amplifies the sensed ground fault signal and applies the amplified signal to a window comparator which compares it to positive and negative reference signals. If either reference value is exceeded, a trip signal is generated. A common type of ground fault detection circuit is the dormant oscillator detector. This detector includes a first sensor coil through which the line and neutral conductors of the protected circuit pass. The output of the first sensor coil is applied through a coupling capacitor to the above-described operational amplifier followed by a window comparator. A line-to-ground fault causes the amplified signal to exceed the reference value and generates a trip signal.
The dormant oscillator ground fault detector includes a second sensor coil through which only the neutral conductor passes. A neutral-to-ground fault couples the two detector coils causing the amplifier to oscillate which also results in generation of a trip signal.
It has been found that wide band noise induced by load related switching phenomena such as is caused by unplugging inductive appliances causes false tripping of the ground fault interrupter.
Commonly owned U.S. patent application Ser. No. 023,435 filed on Feb. 26, 1993 discloses a ground fault circuit interrupter of the dormant oscillator type which utilizes the output of the second sensor coil to detect sputtering arc faults. This is accomplished by bandwidth limiting the di/dt sensor signal and comparing it to appropriate thresholds such as in a window comparator. Sputtering arc fault currents are generally smaller in magnitude than line-to-ground fault currents, but are intermittent, so that detection of successive excursions above the thresholds provides an indication of a sputtering arc fault.
There is a need for a ground fault interrupter which does not generate a false trip in response to wide band noise in the protected circuit.
There is also a need for such a ground fault circuit with improved immunity to wide band noise which also responds to sputtering arc faults.